This invention relates to an antenna arrangement, and is particularly concerned with microwave antenna arrangements.
According to this invention, a microwave antenna arrangement comprises a microwave antenna structure characterized by a fine wire dielectric positioned in front of said microwave antenna structure so that microwaves transmitted or received by said antenna structure pass through said dielectric, which has a dielectric constant of less than unity at microwave frequencies and a plasma frequency below that of said microwaves.
By a fine wire dielectric it is meant an array of thin elongate electrical conductors which exhibits a dielectric constant of less than unity below a plasma frequency. It has been shown that a fine wire dielectric behaves like a low density plasma of very heavy charged particles with a plasma frequency in the GHz range, see xe2x80x9cLow frequency plasmons in thin-wire structuresxe2x80x9d by J. B. Pendry, A. J. Holden, D. J. Robbins and W. J. Stewart, J Phys: Condensed Matter 10 (1998) 4785-4809.
The combination of the fine wire dielectric and the antenna structure enables the operation and performance of the antenna to be modified in various ways. By arranging that the dielectric constant is between zero and unity over the operational frequency band of the antenna structure, the apparent size or aperture of a radiating or receiving antenna element is increased, thereby permitting a physically narrower radiation beam to be produced resulting in an enhanced performance.
The fine wire dielectric may take various forms. For ease of manufacture, it is preferred that it consists of a plurality of spaced apart planes, with parallel fine wires lying in each plane, and with the direction of the wires alternating by 90xc2x0 for successive planes. Alternatively, the fine wires can comprise a mesh in which two sets of parallel wires lie in a common plane so as to interconnect at their crossing points, and furthermore the fine wire dielectric can take the form of a three-dimensional structure, by providing an array of wires at right angles to the planes of these two sets, thereby forming a three-dimensional lattice. Instead, the dielectric can comprise short individual wires at right angles to the plane of the dielectric, so that it has a xe2x80x9chairbrushxe2x80x9d like structure.
The use of the invention permits an antenna arrangement to be constructed in which antenna structures having different operational frequencies physically overlap. For example, an outer, lower frequency antenna structure can be transmissive of higher frequencies received or transmitted by a high frequency antenna mounted behind it. In this case, the dielectric constant is arranged to have a negative value in the low frequency band, so that the dielectric is non-transmissive of the lower frequencies.